CDC - Seasonal Influenza (Flu) - 2009-10 Influenza (Flu) Season

2009-10 Influenza (Flu) Season
Questions & Answers about the 2009–2010 Flu Season

What has this flu season been like?
Flu seasons are unpredictable in a number of ways, including when they begin, how severe they are, how long they last and which viruses will spread. There were more uncertainties than usual going into this flu season (2009-2010), because of the emergence of the 2009 H1N1 influenza virus (previously called "novel H1N1" or "swine flu"). This virus caused the first influenza pandemic (global outbreak of disease) in more than 40 years.

The United States experienced its first wave of 2009 H1N1 pandemic activity in the spring of 2009, followed by a second wave of 2009 H1N1 activity in the fall. The 2009-2010 flu season began very early, with 2009 H1N1 viruses predominating and causing high levels of flu activity much earlier in the year than during most regular flu seasons. In addition, the fall wave of 2009 H1N1 activity (like the spring) was relatively more severe among people younger than 65 than seasonal flu usually is and relatively less severe among people older than 65 than seasonal flu usually is. Activity so far this season peaked in October and then declined quickly to below baseline levels in January. The early rise in flu activity in October is in contrast to past influenza seasons. Influenza activity usually peaks in January, February or March. (See graph of peak influenza activity by month in the United States from 1976-2009.)

There are still uncertainties surrounding the rest of this flu season, including the possibility of the circulation of seasonal influenza viruses and ongoing circulation of 2009 H1N1 viruses. In past pandemics, flu activity has occurred in waves and it’s possible that the United States could experience another wave of flu activity. In addition, sporadic cases of influenza may also be detected in the summer.

When did the flu season peak?
The weekly percentage of outpatient visits for influenza-like illness (ILI) peaked at the end of October at 7.7%, a level higher than the three previous influenza seasons, as reported by the U.S. Outpatient ILI Surveillance Network (ILINet). This percentage decreased to 1.8% at the beginning of January, 2010 The number of states reporting widespread influenza activity dropped from 49 at the end of October to zero at the beginning of January 2010. While states are not reporting widespread influenza, most states are still reporting some influenza activity.

Is the flu season over?
Although influenza activity has declined recently, additional waves of influenza activity due to 2009 H1N1, seasonal influenza viruses, or both, may occur later this influenza season. Flu season can last as late as May. Even if the U.S. doesn't experience a sharp increase in influenza activity during the remaining winter or spring (another "wave" of influenza), continued low level circulation of influenza viruses may continue during this time.

How is severity characterized?
The overall health impact (e.g., infections, hospitalizations and deaths) of a flu season varies from year to year. Based on available data from U.S. influenza surveillance systems monitored and reported by CDC, the severity of a flu season can be judged according to a variety of criteria, including:

The number and proportion of flu laboratory tests that are positive;
The proportion of visits to physicians for flu-like illness;
The proportion of all deaths that are caused by pneumonia and flu;
The number of flu-associated deaths among children; and
The flu-associated hospitalization rate among children and adults.
A season's severity is determined by assessing several of these measures and by comparing them with previous seasons.

How effective is the seasonal flu vaccine?
The effectiveness of flu vaccines can vary and depends in part on the match between the viruses in the vaccine and the flu viruses that are circulating in the community. If these are closely matched, vaccine effectiveness (VE) is higher. If they are not closely matched, VE can be reduced. During well-matched years, clinical trials have shown VE between 70% and 90% among healthy adults. For more information about seasonal flu vaccine effectiveness, visit "How Well Does the Seasonal Flu Vaccine Work?"

abrir aquí para acceder al documento CDC completo del cual se reproduce una mínima parte:
CDC - Seasonal Influenza (Flu) - 2009-10 Influenza (Flu) Season